This PDF file contains the front matter associated with SPIE Proceedings Volume 7603, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

We present a full band Monte Carlo study of high field carrier transport and impact ionization properties of wurtzite ZnO. The proposed model is based on an accurate electronic structure calculated with a nonlocal empirical pseudopotential method and a phonon dispersion determined with density functional theory. The model includes the full details of the lowest eight conduction bands and the top six valence bands derived from the empirical pseudopotential method and a numerically calculated impact ionization transition rate based on a wave-vector dependent dielectric function. The carrier-phonon interaction is treated using the rigid pseudoion formalism, thus removing adjustable parameters such as deformation potential coefficients. Electric-field-induced interband transitions are included in the model by the direct solution of the time-dependent multiband Schrödinger equation. The hole ionization coefficient is found to be very low compared to the electron ionization coefficient. The low ratio of hole and electron ionization coefficients k = β/α holds the promise for high speed and low noise avalanche photodetection in the ultraviolet spectral range.

We report on measurements and calculations of the ultrafast exciton relaxation dynamics in ZnO. Time-resolved differential reflectivity measurements of bulk ZnO were performed as a function of excitation wavelength. Bi-exponential decays of the A and B exciton states are observed with a fast (~2-5 ps scale) and a slower (~50-100 ps scale) component, which depend strongly on excitation wavelength. Theoretical calculations based on a multi-state, coupled rate equation model were directly compared with the experiments to account for the rapid scattering between the A and B valence bands. Results show that the inter-valence band scattering is most likely not responsible for the fast initial relaxation. Instead our results show that carrier diffusion can play an important role in explaining the initial fast relaxation.

The optical, structural, and electrical properties of zinc oxynitrides grown by reactive rf magnetron sputtering are investigated. Oxygen and nitrogen compositions in the layers varied from 55% to 0% and 3% to 44%, respectively, thus ranging from N-doped ZnO to pure Zn₃N₂. A ZnO layer is included atop of the films to prevent N loss. Rutherford backscattering shows the increase of Zn as well as the reduction of O in the alloy as the N content increases. X-ray diffraction measurements evidence the deformation of the wurtzite lattice as the N is introduced, showing a strong impact on the c parameter of the unit cell. Intermediate N contents reduce the crystal quality giving rise to quasi-amorphous layers. Optical transmission measurements were used to determine the absorption cut-off wavelength as a function of N content. The type of spectrum obtained for a N content of 4% along with the observed morphology seem to point out to the existence of two phases. For similar O and N concentrations, the X-ray pattern yields features of a new crystal phase. For these compositions, electrical characterization exhibits a minimum resistivity of 0.42 Ω cm.

Spectacular breakthroughs have been achieved in optoelectronics with the use of ZnO as a source for light-emitting diodes (LED) and quantum wells (QWs). In particular, atomically flat surfaces were obtained in Zn-polar growth, which led to the fabrication of Mg-rich Mg_0.37Zn_0.67O/ZnO QWs with sharp heterointerfaces between MgZnO and ZnO. μ-PL spectroscopy revealed that excitons were efficiently confined in the wells at room temperature. Excitonic emissions from Zn-polar QWs did not have a linear polarization effect, although polarized lights were clearly observed in M-nonpolar Mg_0.12Zn_0.88O/ZnO QWs. From the optical selection rule, the polarized lights of excitonic emissions were based on A- and C-excitonic transitions under E⊥c and E//c configurations. Furthermore, the anisotropic surface morphology was self-organized on the M-nonpolar ZnO layer surfaces, which allowed examination of the relationship between electron transport and surface morphology. The observed transport anisotropy correlated with the surface morphology. On the other hand, Zn-polar QWs resulted in isotropic electron transport because of two-dimensional surfaces. In this presentation, we introduce the detailed growth and various properties of Zn-polar and M-nonpolar QWs.

There is general agreement that the future production of electric energy has to be renewable and sustainable in the long term. Photovoltaic (PV) is booming with more than 7GW produced in 2008 and will therefore play an important role in the future electricity supply mix. Currently, crystalline silicon (c-Si) dominates the market with a share of about 90%. Reducing the cost per watt peak and energy pay back time of PV was the major concern of the last decade and remains the main challenge today. For that, thin film silicon solar cells has a strong potential because it allies the strength of c-Si (i.e. durability, abundancy, non toxicity) together with reduced material usage, lower temperature processes and monolithic interconnection. One of the technological key points is the transparent conductive oxide (TCO) used for front contact, barrier layer or intermediate reflector. In this paper, we report on the versatility of ZnO grown by low pressure chemical vapor deposition (ZnO LP-CVD) and its application in thin film silicon solar cells. In particular, we focus on the transparency, the morphology of the textured surface and its effects on the light in-coupling for micromorph tandem cells in both the substrate (n-i-p) and superstrate (p-i-n) configurations. The stabilized efficiencies achieved in Neuchâtel are 11.2% and 9.8% for p-i-n (without ARC) and n-i-p (plastic substrate), respectively.

Fluorine doped tin oxide (FTO) and aluminum doped zinc oxide (AZO) were systematically investigated as alternatives to indium tin oxide (ITO) for canonical poly(3-hexylthiophene) (P3HT) + [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) polymer bulk heterojunction (BHJ) solar cells. Devices made with FTO performed twice as well as devices made with ITO, establishing FTO as a suitable, low-cost ITO replacement. Ozone treatment was shown to be a critical enabling element for both FTO and AZO. X-ray photoelectron spectroscopy (XPS) and device characteristics were used to explain the effect of ozone treatment and the origin of open circuit voltage.

ZnO-based compounds are of interest as buffer layers in Cu(In,Ga)Se₂ (CIGS) solar cells, due to the ability to change the electrical and optical properties of ZnO by addition of other elements. The device structure of a CIGS solar cell is; soda-lime glass/Mo/CIGS/buffer layer/ZnO/ZnO:Al. This contribution treats growth and characterization of Zn_1-xMg_xO and Zn(O,S) on glass substrates and as buffer layers in CIGS solar cell devices. The ZnO-based compounds are grown by atomic layer deposition at deposition temperatures below 200 °C using metal-organic precursors.

Thin film solar cells based on Cu(In,Ga)Se₂, called CIGS, is one of the most promising technologies for low cost, high efficiency photovoltaics. The CIGS device is composed of four layers; molybdenum back contact, CIGS p-type absorber, n-type buffer layer and doped ZnO top contact. The most common buffer layer is CdS, however it is desirable to find a Cd-free, large band gap alternative. In this paper, the use of ZnO-based buffer layers deposited by atomic layer deposition, ALD is described. Efficiencies of over 18% are shown by using Zn(O,S) or (Zn,Mg)O by ALD followed by sputtered ZnO:Al. The role of the conduction band alignment across the heterojunction is discussed, and results for large band gap CuGaSe₂ absorbers are presented. In addition, light-soaking effects for devices with (Zn,Mg)O-based buffer layers are related to measurements of persistent photoconductivity of ALD-(Zn,Mg)O thin films.

Transparent conductive oxides have an important role of thin-film solar cell to the side of cost and performance. Especially, an Al doped ZnO film is a very promising material for thin-film solar cell fabrication because of the easy synthesis method as well as the cheap cost induced on the wide availability of its constituent raw materials. The Al-doped ZnO films were prepared by metal organic chemical vapor deposition using a diethylzinc, water vapor and trimethylaluminum (TMA). The introduction of TMA doping source has a great influence on the electrical resistivity and diffused light to wide wavelength range. The haze factor of Al doped ZnO achieved 43 % at 600 nm without additional surface texturing process by simple TMA doping. In this study, the choice of AZO TCO allows the achievement of energy conversion efficiencies to 7.7 %.

The results of a combined study of Raman scattering, IR absorption, photoluminescence, and photoconductivity on ZnO are presented. Two shallow donors - hydrogen at the bond-centered lattice site, H_BC, and hydrogen bound in an oxygen vacancy, H_O, - were identified. Donor H_BC has an ionization energy of 53 meV. The recombination of an exciton bound to H_BC gives rise to the 3360.1±0.2 meV photoluminescence line. The 1s → 2p donor transition at 330 cm^-1 is detected in the Raman scattering and photoconductivity spectra. The stretch mode of the associated O-H bond is detected in IR absorption at 3611 cm^-1. The H_O donor in ZnO has an ionization energy of 47 meV. The excitonic recombination at H_O leads to the previously labeled I₄ line at 3362.8 meV. Photoconductivity and Raman spectra reveal the 1s → 2p donor transition at 265 cm^-1. It is shown that H_BC migrating through the ZnO lattice forms electrically inactive interstitial H₂. Vibrational modes of H₂, HD, and D₂ were identified at 4145, 3628, and 2985 cm^-1, respectively. These results suggest that interstitial H₂ is responsible for the "hidden" hydrogen in ZnO.

Modifying the properties of ZnO by means of incorporating antimony, arsenic or phosphorus impurities is of interest since these group V elements have been reported in the literature among the few successful p-type dopants in this technologically promising II-VI compound. The lattice location of ion-implanted Sb, As, and P in ZnO single crystals was investigated by means of the electron emission channeling technique using the radioactive isotopes ¹²⁴Sb, ⁷³As and ³³P and it is found that they preferentially occupy substitutional Zn sites while the possible fractions on substitutional O sites are a few percent at maximum. The lattice site preference is understandable from the relatively large ionic size of the heavy mass group V elements. Unfortunately the presented results cannot finally settle the interesting issue whether substitutional Sb, As or P on oxygen sites or Sb_Zn-2V_Zn, As_Zn-2V_Zn or P_Zn-2V_Zn complexes (as suggested in the literature) are responsible for the acceptor action. However, the fact that the implanted group V ions prefer the substitutional Zn sites is clearly a strong argument in favour of the complex acceptor model, while it discourages the notion that Sb, As and P act as simple "chemical" acceptors in ZnO.

We study the electronic and magneto-transport properties of multiferroic oxide-based structures and explore their potential for spintronic applications. In particular, we point out the possibility of using the two dimensional electron gas (2DEG) formed at the interface of helimagnetic oxides as a spin-field-effect transistor and a flash memory device. The operation of this device relies on the fact that the topology of the multiferroic oxide local magnetic moments results in a resonant momentum-dependent effective spin-orbit interaction acting on 2DEG. The spin polarization dephasing is strongly suppressed which is crucial for functionality. The effective spin-orbit interaction and the carrier spin precession phase depend linearly on the magnetic spiral helicity which, due to the magnetoelectric coupling, is electrically controllable. We also consider helical multiferroic tunnel junctions with a normal metallic layer as the bottom electrode and a ferromagnetic layer as the other electrode. It is shown that the tunnel-magneto-resistance is spatially dependent and is controllable via an external electric field.

Atomic layer epitaxy of zizc oxide (ZnO) and titanium dioxide (TiO₂) have been applied to compose superlattice film devices such as soft x-ray multilayers. The reason why oxide films are chosen, is that oxygen of oxide films has transparency for the "water-window" (λ=2.332-4.368 nm) wavelengths. Actually, we researched the fabrication of TiO₂/ZnO multilayer mirrors and then found that these multilayer films provided high reflection in the wavelength region. Our theoretical calculation indicated that multilayer mirror could have the high reflectance of nearly 50% at the wavelength of 2.73 nm and at the incidence angle of 18.2° from the normal incidence. TiO₂/ZnO films were grown on the c-plane (0001) sapphire substrate by means of atomic layer epitaxy (ALE) technique, which involved the alternate reactions of Zn(CH₂CH₃)₂ (DEZ) and H₂O, and Ti(Cl)₄ (TCT) and H₂O. Our experimental results indicated that thin Wurtzite ZnO (0001) and Rutile TiO₂(200) films were grown epitaxially on c-plane (0001) sapphire substrates at 450°C with self-limiting mechanism. The 10-bilayer TiO₂/ZnO multilayer indicated high soft X-ray reflectivity of around 30%.

Core shell BaTiO₃ based particles sintered using advanced processes provide a high control of grain boundaries in bulk composites. As a result, supercapacitor behavior was evidenced which came from the balance between inner grain conductivity and grain boundary dielectric barrier. Thanks to the core-shell structure of the starting particles, improved control of the effective dielectric parameters can be achieved.

Thin films of strontium titanate, as well as its component oxides, have been of great interest due to their applicability in electronic devices. In this manuscript we review our work to examine the growth of SrO, TiO₂, and SrTiO₃ (STO) thin films on STO using classical molecular dynamics simulations. In particular, the simulations considered the deposition of SrO and TiO₂ molecules and stoichiometric STO clusters at incident energies of 0.1, 0.5, and 1.0 eV/atom onto the (001) surface of STO. The role of surface termination layer (SrO vs. TiO₂), incident energy, and incident particle size and deposition scheme in the case of STO deposition were investigated. In the case of SrO deposition, smooth, ordered films were produced at all incident energies considered and for both surface terminations. In contrast, in the case of TiO₂ deposition, three-dimensional islands were formed under all the conditions considered. The predicted growth modes were shown to be a consequence of the mobility and interaction energy of each particle (SrO or TiO₂) with the surface. In the case of STO thin film growth three deposition schemes were explored: (i) alternating particle deposition (APD), (ii) alternating monolayer deposition (AMD), and (iii) cluster deposition. For APD, a beam of alternating SrO and TiO₂ molecules was deposited on the (001) surface of STO with incident kinetic energies of 0.1, 0.5 or 1.0 eV/atom. AMD consisted of the deposition of alternating monolayers of pure SrO and TiO₂, where both had incident energies of 1.0 eV/atom. SrTiO₃ cluster deposition considered the deposition of 1, 2, 3, and 4-unit STO stoichiometric clusters with incident energies of 1.0 eV/atom. On the whole, some layer-by-layer growth was predicted to occur on both SrO- and TiO₂-terminated STO for each type of deposition scheme.

We report on structural, magnetic, and transport properties of La_xMnO_3-δ thin films, epitaxially grown on SrTiO₃ substrates by molecular beam epitaxy deposition technique. We varied the La/Mn ratio by changing the evaporation rate of the single-element diffusive cells. However, the oxygen content of La_xMnO_3-δ thin films was varied by post-annealing them in air and/or vacuum, by changing the annealing temperature and the time of post-annealing process. Optimal oxygenated La_0.88MnO_3-δ (LMO) films show extremely high metal insulator transitions temperature T_MI ~ 380K and magneto-transport similar to those found in strontium-doped La_1-xSr_xMnO₃ manganites compounds. Magnetic measurements confirm the formation of a ferromagnetic phase at Curie temperature T_c of about 360K. All these findings clearly demonstrate that the lanthanum deficiency, with respect to bivalent cation-substitution, is a very efficient way to hole-dope manganites.

Vanadium dioxide (VO₂) is a strongly-correlated electron material with a well-known semiconducting to metallic phase transition that can be induced thermally, optically, or electrically. When switched to the high-temperature (T > 68°C) metallic phase, the greatest contrast in the optical properties occurs at wavelengths in the near-to-mid-infrared and beyond. In the visible to near-infrared, however, upon switching for wavelengths between ~500-1000 nm, VO₂ transmits more light in the metallic phase. In this paper, we report studies of the effect of near-IR irradiation (785 nm) on lithographically prepared arrays of gold nanoparticles (NPs) covered with a thin film of VO₂ and find that the presence of the NPs substantially lowers the laser threshold for low-power induction of the phase transition. Hybrid Au::VO₂ structures were created by coating lithographically prepared arrays of gold nanoparticles (NPs) (diameters 140 and 200 nm, array spacing 450 nm) with 60 nm thick films of VO₂ by pulsed laser deposition. Due to resonant absorption of the Au particle-plasmon resonance (PPR) at 785 nm, a temperature-dependent shift in the PPR can be generated by switching the VO₂ from one phase to another. We have measured the switching behavior of VO₂ and Au::VO₂ structures using shuttered CW laser irradiation in order to study both optical and thermal mechanisms of the phase transition. Transient absorption measurements using a shuttered 785 nm pump laser corresponding to the PPR resonance of the Au NPs and 1550 nm CW probe show that the presence of the Au NPs lowers the threshold laser power required to induce the phase transition.

In this paper, we present the current research efforts on the atomic layer deposition (ALD) ZnO based TFT devices carried out in our laboratory. ZnO thin film deposition was carried out by two different ALD processes; thermal ALD using water as a reactant and plasma-enhanced ALD using oxygen plasma as a reactant. The film properties were comparatively studied showing large difference in terms of electrical properties. For thermal ALD ZnO, carrier concentrations were too high to fabricate well-operated ZnO TFTs. To control the carrier concentration, nitrogen doping was utilized based on NH4OH reactant. Meanwhile, for PE-ALD, highly resistive films were obtained at low growth temperature below 200 °C. To reduce the resistivity to a proper level for the fabrication of TFTs, UV-light exposure was used. At properly controlled conditions, high performance TFT devices were fabricated based on these processes. ZnO TFTs were also fabricated on flexible substrates and the initial research was carried out on the effects of device bending on device properties.

Complementary use of p-type organic and n-type oxide semiconductors is presented. First, we demonstrated complementary circuits using low-voltage operating high performance pentacene and amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs). The field-effect mobilities of the pentacene and a-IGZO transistors are 0.6 and 17.1 cm²/V s, respectively at an operating voltage of 10 V. A complementary inverter composed of these transistors exhibits good voltage transfer characteristics with a high gain of ~56. A five-stage ring oscillator with the inverters yields an output frequency of 200 Hz at 10 V, corresponding to a propagation delay of 1 ms. Second, together with the electrical device, we demonstrated an optoelectronic device, light-emitting diodes (LEDs), using organic/oxide hybrid junctions. The hybrid p-n junction LEDs are composed of N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4'-diamine (α-NPD) and sputtered ZnO. Similar with conventional p-n junction diodes, the hybrid junction shows a good current rectification and electroluminescence (EL) under forward bias. We found that the EL bands from the device agree well with the photoluminescence peaks from α-NPD and ZnO, implying the radiative recombination of injected charges occurs in both components of the junction.

Oxide semiconductors became one of the potential elements for large area electronics such as a channel for thin film transistors. Optical and electrical properties were modified by alloying or doping of several oxide materials; In₂O₃, ZnO, Ga₂O₃, and SnO₂. The excellent properties achieved at the ternary or quaternary alloys could be explained by the role of each materials as a carrier controller, a conduction path, and etc. The metal oxide semiconductors were generally deposited by vacuum process but recently, alternative ways, like a sol-gel or an ink-jet printing, are suggested. In this review, diverse approaches on oxide semiconductors are shown, and an in-depth discussion of the optical and electrical properties alternation in metal oxide alloy fabricated by various methods is given.

Reported herein is a nonvolatile n-type floating gate memory paper field-effect transistor, emphasizing the role of the paper structure and properties on the device performance recorded such as in the high capacitance per unit area at low frequencies (>2.5 μFcm^-2) and so on the set of high charge retention times achieved (>16000 hours). The device was built via the hybrid integration of natural cellulose fibers, which act simultaneously as substrate and gate dielectric, using amorphous indium zinc and gallium indium zinc oxides respectively for the gate electrode and channel layer. This was complemented by the use of continuous patterned metal layers as source/drain electrodes.

Enhancement-mode TFTs based on amorphous InGaZnO channel were fabricated on paper, glass or plastic substrates at low temperature (< 100°C). The TFTs operated in enhancement mode and showed low operating voltages of 0.5-2.5 V, drain current on-to-off ratios of ~ 10⁵, sub-threshold gate-voltage swing of 0.25-0.5 V.decade^-1, and high saturation mobilities of 5-12 cm².V^-1.s^-1. The devices exhibited small shifts during 1000 hours aging time at room temperature. Significant challenges remain, including improving the stability of the devices under bias, lowering the operating voltages, replacing metal contacts with conducting polymers that should be more resistant to cracking on rolling-up of flexible substrates and developing large-area printing processes that are compatible with manufacturing these devices on very large areas.

Thin film transistor (TFT) device structure with transparent conductive oxide semiconductor is proposed for the photosensor application. The adoption of TFT-based photosensor device also is promising to be integrated with pixel-array circuits in a flat panel display and realize the system-on-panel (SoP) concept. The photosensitive TFT device can be applied to sense the ambient light brightness and then give the feedback to the backlight system adjusting the backlight intensity for the power-saving green displays. In this work, we studied the photosensitivity of amorphous indium zinc oxide (a-IZO) TFT to ultraviolet light. The a-IZO-based semiconductors have been paid much attention due to their uniform amorphous phase and high field-effect carrier mobility characteristics. The obvious threshold voltage shift was observed after light illumination, and exhibited slow recovery while returning to initial status after removing the light source. This mechanism for the photoreaction is well explained by the dynamic equilibrium of charge exchange reaction between O_2(g) and O₂^- in the backchannel region of IZO-based films. An electrical trigger using charge pumping method is used to confirm the proposed mechanism and accelerate photoreaction recoverability for the first time. Using knowledge of photoreaction behavior, an operation scheme of photosensing elements consist of a-IZO TFTs is also demonstrated in this paper.

Thin Film Transistors (TFT) were made by growing ZnO on Si₃N₄/SiO₂/Si (111) substrates by pulsed laser deposition. X-ray diffraction and scanning electron microscope studies revealed the ZnO to have a polycrystalline wurtzite structure with a smooth surface, good crystallographic quality and a strong preferential c-axis orientation. Transmission studies in similar ZnO layers on glass substrates showed high transmission over the whole visible spectrum. Electrical measurements of a back gate geometry FET showed an enhancement-mode response with hard saturation, mA range Id and a V_ON ~ 0V. When scaled down, such TFTs may be of interest for high frequency applications.

Among wide bandgap materials that are sensitive to photons in the ultraviolet (UV) region, ZnO is a promising photonic material because of its unique optoelectronic properties. Based on the lateral interdigitated back-to-back Schottky contact structure on ZnO film, metal-semiconductor-metal (MSM) photodetectors have substantially lower parasitic capacitance compared with vertical p-i-n photodetectors, which leads to a very high speed photodetection. In this paper, we report optical characteristics of MSM ZnO UV photodetectors for which ZnO films were fabricated by hybrid beam deposition. An annealing process was used in oxygen ambient. The MSM ZnO photodetector consists of two interdigitated electrodes both with Ti/Au metals on an n-type ZnO thin film. The electrodes on the photodetector are finger-shaped. We found that the annealing process decreases contact resistance and photoresponse time. The possible mechanism of annealing process is the removal of surface defects created in the fabrication process. A sublinear power dependence of photocurrent reveals the existence of a light induced space charge region inside the ZnO film. The device displays fast pulse response with a very short rise time and a relatively long relaxation time with applied bias. The exponential decay tail indicates an RC type time response.

Oxide based compounds have been of increasing interest for wide bandgap, deep ultraviolet optoelectronics. While high Al content AlGaN has enabled many UV-DUV technologies, it suffers inherent drawbacks including difficulty achieving increasing Al incorporation, high threading dislocation densities and challenges in bandgap engineering due to polarization and piezoelectric effects. Here we present two wide bandgap cubic oxide compounds, ZnMgO and NiMgO, that offer advantages over AlGaN for deep ultraviolet (DUV) applications. Ni_xMg_1-xO and Zn_xMg_1-xO are both direct band gap, cubic rocksalt (B1) semiconductors with bandgaps in the UV-DUV spectral regions, offering alternatives without the aforementioned drawbacks associated with AlGaN. Here we present Ni_xMg_1-xO and Zn_xMg_1-xO thin films grown by plasma-assisted MBE on lattice matched MgO substrates as a novel means by which to realize DUV detection devices. In both systems we have shown the films to exhibit abrupt, continuously tunable absorptions edges over their respective bandgap ranges. Ni_xMg_1-xO films were varied compositionally from x=0 to 1, realizing bandgaps from 3.5 to 7.8 eV. Zn_xMg_1-xO films were similarly varied over the entire B1 range of the ternary (0<x<0.42) and show bandgap tunability from ~5 to 7.8 eV. All films are characterized through Rutherford backscattering (RBS), x-ray diffraction (XRD), atomic force microscopy (AFM) measurements and optical transmission. Significantly, we have successfully fabricated solar blind detectors in both categories and highlight the results from Ni_xMg_1-xO here.

InGaN/GaN layers were grown on ZnO-buffered Si (111) substrates by metalorganic vapour phase epitaxy (MOVPE). The dissociation of ZnO observed during conventional MOVPE growth of InGaN/GaN was combated through the use of a low pressure/temperature MOVPE approach with N₂ as a carrier gas and dimethylhydrazine added to the ammonia (nitrogen precursor) in order to enhance the concentration of atomic nitrogen at relatively low temperature. Electron Microscopy of cross-sections, High Resolution X-Ray Diffraction (HR-XRD), secondary ion mass spectroscopy and cathodoluminescence studies suggested that single phase wurtzite InGaN layers with between about 17.5 and 21.5% indium were grown epitaxially, with no evidence of back-etching of the ZnO templates. HR-XRD revealed highly pronounced c-axis texture for both the InGaN/GaN and ZnO. Immersion in dilute nitric acid dissolved the ZnO such that the InGaN/GaN could be lifted-off from the substrate.

In the past few years, we have fabricated nanoscale La_0.7Sr_0.3MnO₃ periodic arrays with unique optical and magnetic properties successfully. These periodic patterns were made by La_0.7Sr_0.3MnO₃ resist that can be developed under a nontoxic and environmental friendly manner using pure water. The resist is also capable to exhibit both positive and negative resist behaviors depending on the electron beam dosage. Thus, these special characteristics are used to fabricate and tune periodic structure thin film having controlled optical reflectance properties in the wavelength of 300 nm to 800 nm with one fixed design electron beam pattern by simply changing the electron beam dosage only. Additionally, the magnetization of La_0.7Sr_0.3MnO₃ patterns can be enhanced by post sintering the sample at 900 °C after electron beam irradiation. Therefore, our study provides a one-step, simple, and convenient alternative technique for the fabrication of tunable optical structure and nanoscale magnetic patterns, which form the building blocks for the study of optoelectronic and magnetic devices in nanoscale periodic arrays.

Multi-layered sensor based on nanostructured ruthenium oxide (RuO₂) sensing electrode (RuO₂-SE) deposited on platinized alumina substrate and capable of being coupled with a simple turbidity sensor has been evaluated for long-term pH stability during an 18-month continuous trial. The sensor is designed to detect the main parameters of water quality: pH and dissolved oxygen (DO) over a temperature range of 9-30°C. The chemical composition and morphology of the RuO₂-SE was investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). It was found that both the morphology and surface compositions of nanostructured RuO₂-SEs did not change significantly after 18-months trial. They maintain their high sensitivity to adsorption of superoxide ions (O₂^-) despite heavy bio-fouling. Sensors with RuO₂-SE have demonstrated a stable Nernstian response to pH from 2.0 to 13.0 and were also capable of measuring DO in the range of 0.5 - 8.0 ppm. The measurement results show very good linearity, and excellent reproducibility was obtained during a trial. The Nernstian slope was approximately 58mV/pH at a temperature of 23°C. Although RuO₂-SEs have been shown to exhibit very good response time to pH changes, within few seconds at a temperature of 23°C, as the water temperature cooled down the sensor response time increased significantly to about 8-10 min or longer at a temperature of 9°C. The influence of hydrogen ion (H⁺) diffusion in nanostructured RuO₂ films on the output emf drift during pH measurements was also investigated.

Using lithographic patterning techniques, normally we aim for the integration of structural elements into a more complex apparatus, which can be at various length scales, for example hand-held equipment. Nanoscale fabricated pillars, holes or wires have shown unique properties already and ordering these in specific arrangements results in novel phenomena normally not present in natural occuring materials. Such materials are called nanoarrays. Engineered nanoarrays belong therefore to the class of metamaterials. One example of a metamaterial is a material with a negative refractive index created by design of artificial structure. These exciting material properties bring about also new opportunities for applications. A functional device or system demanding some level of ordering in a material also requires a carefully designed manufacturing process. Here, we will present an overview of nanolithographic techniques for oxide nanoarrays. Bio-inspired templated nanoarrays will be described in perspective to other nanolithography techniques. These nanostructures can deliver new functionality, too. Moreover, (nano)structured materials can deliver specific functionality at the interface with biological material. Developing these materials, subsequently, we can look for medical applications where the properties of oxide nanoarrays are explored. Photonic crystals, for example, can be applied in medical diagnostic devices. In this paper, therefore oxide nanoarrays are introduced and the emerging technology for modification and tuning of medical device performance utilizing oxide nanoarrays is discussed.

The wide band gap and unique photoluminescence (PL) spectrum of nanocrystalline zinc oxide (nano-ZnO) make it useful for a variety of photonics and sensor applications. Toward the goal of modifying the electronic structure and optical properties of nano-ZnO, nanorods were functionalized with electron withdrawing organosilanes, 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDS) and pentafluorophenyltriethoxysilane (PFS), and a partially conjugated heterobifunctional molecule, p-maleimidophenyl isothiocyanate (PMPI). Fourier transform infrared (FTIR) spectroscopy and x-ray photoelectron spectroscopy (XPS) confirmed the presence of the modifiers on the nano-ZnO surface and verified covalent attachment. PL spectroscopy was performed to evaluate the influence of the modifiers on the nano-ZnO inherent optical behavior. An increase in the nano-ZnO near-band edge emission (UV) was evident for the organosilane modifiers, despite their differing electronic structures, while the defect emission (visible) remained unchanged. However, surface modification with the non-silane modifier PMPI resulted in unaltered UV and visible emission intensity. The varying influence of the modifiers may be due to the absence of a silane group in the PMPI, allowing for more efficient electron transport to the modifier. The influence of size/shape of the nanocrystalline ZnO was also examined by reacting spherical nanoparticles with PFDS. Preliminary results indicate that PFDS modification of the nanospheres resulted in similar PL behavior as the nanorods; although, the inherent PL of the spheres differs from the nanorods. These studies will elucidate the role of modifier structure on surface-modified nano-ZnO optical behavior, so that optical tailoring of the nano-ZnO inherent PL can be realized.

Photoconductors based on wide band gap semiconductors are potential devices for UV light detection due to internal photoelectrical gain and fabrication simplicity. Photoresponses of photoconductors based on GaN and ZnO show high values in UV range under large biases and relatively low values in visible range. Although photoresponse of ZnO photoconductors is similar to that of GaN-based photoconductors, mechanisms of photoconductance between two materials are very different. This difference can be found in optical power dependence of photocurrent and I-V characteristics, and has an impact on device design. In this paper we report experimental studies of photoresponse for newly developed ZnO photoconductors. The ZnO film was grown on a 6H-SiC substrate by hybrid beam deposition. The photoconductor device is formed with interdigitated finger-shaped Ti/Au ohmic contacts on the ZnO film. Electrical characteristics, spectral photoresponse, and persistence properties were studied for the device under variable biases. We find that there are at least three mechanisms involved in the device. At low biases and low incident light power, the photoresponse is mainly due to photocreation. At higher light power and lower biases, the space charge regions are responsible for the photocurrent. At higher biases, the contribution from surface states is dominant.

ZnO thin films were deposited over n⁺ InP (100) substrates by Pulsed Laser Deposition (PLD) technique at 400°C temperature in an oxygen ambient of 75 mTorr followed by Rapid Thermal Annealing (RTA) at the temperatures 450°C, 550°C and 650°C respectively. XRD results revealed that the full width at half maxima (FWHM) of the annealed samples were narrower (0.1836°) compared to that of the as grown sample (0.3264°) for the c-axis oriented ZnO (002) films. A lower strain (~ -0.23%) and less biaxial compressive stress (~ -1.063 GPa) were observed for the annealed samples. AFM images depicted lowest surface roughness of 7.257 nm (root-mean-square) for the film annealed at 550°C. A high absorption coefficient of 28.12 μm^-1 was calculated around 380 nm wavelength from the UV/VIS spectroscopy in reflection mode for the as-grown sample. The optical band gap was calculated to be about 3.23 eV. p-type ZnO film, grown under same condition (annealed at 550°C) over semi insulating InP (100) substrates had a high hole concentration of 2.95X10¹⁹ cm^-3 and Hall mobility of 8.63 cm²/V-s at room temperature. Current Rectification Ratio (I_F/I_R) _|V|=1.5 of 17.2 was measured from the I-V characteristics of the p-ZnO/n⁺-InP heterojunction diode fabricated with the ZnO film annealed at 550°C.

We present the effect of post-annealing on the p-type ZnO:Sb thin film. The ZnO:Sb thin films were grown by pulsed laser deposition (PLD) method. Undoped ZnO thin film was inserted between the ZnO:Sb thin films and GaN template/sapphire substrate to reduce lattice mismatch. The ZnO:Sb thin films grown in the temperature range of 400 °C and 800 °C were all n-type in the as-grown state. However, when post-annealing was performed the film grown at 600 °C and annealed at 700 °C for 60 min showed p-type conductivity with a hole concentration of 1.4 x 10¹⁷ cm^-3 and a reasonable mobility of 6.7 cm²/Vs. The annealed ZnO:Sb thin film grown at a relatively low temperature of 400 °C was highly resistive with a resistivity of 1.95 x 10⁵ Ωcm. This was perhaps due to the low incorporation of Sb in the film. ZnO:Sb thin film grown at a relatively high temperature of 800 °C showed n-type conductivity even after annealing, and the carrier concentration only dropped from 1.32 x 10¹⁷ cm^-3 of the as-grown state to 3.08 x 10¹⁵ cm^-3 after annealing. This may have been due to the formation of Sb compound in ZnO inhibiting the activation of Sb during post-annealing. Therefore, post-annealing of ZnO:Sb grown at adequate temperature is crucial to obtain p-type ZnO thin film.

Titanium dioxide (TiO₂) thin films were prepared by ion-assisted electron-beam deposition on glass at room temperature and were annealed by rapid thermal annealing in O₂ and N₂ gas flow. TiO₂ thin films annealed in N₂ gas flow are (110) rutile phase and (101) anatase phase, but in O₂ gas flow are (110) rutile phase. The optical band gaps of the TiO₂ thin films are increased to 3.281 eV with annealing treatment of 300 ~ 500 °C in O₂ gas flow and to 3.271 eV in N₂ gas flow. However, the band gap begins to decrease to 3.277 eV at the annealing temperature of 600 °C in O₂ gas flow and to 3.257 eV in N₂ gas flow, respectively.

This work investigates the physical properties of the Mg_xZn_1-xO films. Mg_xZn_1-xO films were deposited by RF magnetron sputtering system using a 6 inch ZnO/MgO (80/20 wt%) target. The XPS, Hall measurement, and transparent performance are measured. The XPS results show that there is high Carbon element content on the surface of Mg_xZn_1-xO maybe due to the contamination and the average of the Mg content is about 25 at. %. The XRD results indicate that the appearance of only (111) peaks for as-grown Mg_xZn_1-xO film is a sign of the cubic single phase. In this study, the Mg_xZn_1-xO film show high transparency with transmittances over 90 % in the visible region (400 ~ 700nm) and the sharp absorption edge is visible in UV region due to the Mg content. Therefore, the Hall measurement of Mg_xZn_1-xO films which were deposited at lower RF power show the higher doping concentration, the lower resistivity and higher mobility as a function of the annealing temperatures. The experimental results indicate that Mg_xZn_1-xO film with 800 °C annealing contains more oxygen vacancies which play the role of donor. Since oxygen vacancies generate states in the band gap and cause an increase in conductivity.